Replace the S3-based Nix binary cache with Attic, a multi-tenant
binary cache server with better deduplication and garbage collection.
Changes:
- Install attic-client system-wide from nixpkgs
- Update substituter URL from s3.toph.so to cache.toph.so/toph
- Replace S3 cache keys with Attic cache signing key
- Add attic client config for endurance host
The Attic cache provides the same functionality as the S3 cache but
with improved performance and multi-tenancy support.
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
